Evolution of the ωo phase in a β-stabilized multi-phase TiAl alloy and its effect on hardness

The intermetallic β-stabilized Ti-43.5Al-4Nb-1Mo-0.1B alloy (in at.%), termed TNM alloy, is designed to be used at elevated temperatures, typically up to 750 °C. To understand the evolution of the microstructures during heat treatments and subsequent creep tests, an understanding of the phase transformations and decomposition reactions that occur is necessary. The present study deals with the development and growth mechanism of the ωo phase, which forms in the βo phase during static annealing treatments and creep tests performed at 750, 780 and 800 °C using an applied stress of 150 MPa. In situ high-energy X-ray diffraction experiments were conducted to investigate the decomposition behaviour of the ωo phase during heating as well as to determine its dissolution temperature. High-resolution transmission electron microscopy was used to study the coarsening of ωo grains during creep. The chemical composition of βo and ωo was determined by means of energy dispersive X-ray microanalysis. In particular, the impact of the Mo content on the growth of the ωo grains within the βo matrix was investigated. Additionally, nanohardness measurements in γ, α2, βo and (βo + ωo) grains were performed by cube corner indentation. The results show that βo is the hardest phase in the TiAl-Nb-Mo alloy system when finely dispersed ωo precipitates are present.